1. Field of the Invention
The present invention relates to a circuit for reducing distortion of an FM receiver. More specifically, the present invention relates to a circuit for reducing a noise and distortion unique to an FM receiver which is borne on a moving vehicle, for example.
2. Description of the Prior Art
Conventionally, a so-called car stereo set has been borne on a vehicle. Such a car stereo set comprises an FM receiver. Such FM receiver of a car stereo set gives rise to a noise and distortion because of various external causes. For example, there is a noise caused by a fluctuation of an electric field strength based on a Rayleigh distribution occuring in a weak electric field area. More specifically, in a case where the electric field strength changes from a relatively large state to a small state in a weak electric field area, for example if the input electric field decreases by 30 dB, the S/N ratio is degraded by approximately 55 dB. More specifically, in such a case the noise level (white noise) increases by 55 dB and the increase of the noise level causes an uncomfortable feeling to the ear. There is also a noise caused by so-called multi-path distortion caused by an influence of a reflective wave. It has been known that the former and the latter appear at the rate of approximately 3:7. Conventionally, an approach was taken that in order to mitigate a noise because of the former, i.e. the Rayleigh fluctuation the receiving electric field strength is detected by the output of the intermediate frequency signal circuit, for example, the high frequency region of the demodulated output is suppressed when the electric field strength exceeds a given level and the stereo separation is reduced, whereby the apparent S/N ratio is improved. However, this approach naturally degrades the stereo separation. Furthermore, no particular useful solution has been proposed, in a car stereo set, to a noise because of the multipath distortion. Several solutions also have been proposed in a so-called home stereo set. Some of these are to enhance the directivity of an antenna and rotate the antenna so that the directivity direction may coincide with a transmitter, or to detect a phase delay and a scaler amount of a reflected wave and offset the same with a signal of the same scaler amount and of the opposite phase. However, any of the conventionally proposed solutions for the so-called home stereo set were not able to be applied to a moving FM receiver such as a so-called car stereo set wherein the receiving state changes instantaneously.
Reduction of stereo separation as described above has been disclosed in U.S. Pat. No. 3,539,729, issued Nov. 10, 1970 to D. R. von Recklinghausen and entitled "Apparatus for Reducing Interference in the Transmission of Electric Signals". However, the approach disclosed in the referenced patent does not achieve the effectiveness as obtained by the present invention in response to distortion. Only the second embodiment in the referenced patent is of interest to the present invention. The second embodiment of the referenced patent is directed to an FM stereo receiver. The second embodiment makes use of dependency of a signal to noise ratio upon an antenna input signal intensity, thereby to achieve an additional automatic control in addition to the first embodiment of the referenced patent. For the purpose of the above described additional automatic control of the second embodiment, the output of the FM tuner is applied to a high pass filter. The cutoff frequency of the high pass filter is selected to be the maximum modulation frequency, i.e. 75 kHz. The output of the high pass filter is proportional to the amount of a noise existing in a push-pull fashion in the output of the stereo demodulator. The output of the high pass filter is amplified and is further rectified. The rectified output and the difference voltage are combined and the combined output is applied to a lamp drive circuit. A lamp being driven by the lamp drive circuit is optically coupled to a variable resistor having a resistance value changeable as a function of the intensity of the light from the lamp. The above described variable resistor is interposed between the respective channels of the stereo demodulated output. According to the second embodiment of the referenced patent, when the high frequency noise component included in the FM detector increases, the rectified output correspondingly increases and accordingly the light from the lamp becomes strong. Therefore, the resistance value of the variable resistor optically coupled to the lamp decreases. This means that stereo separation between both channels of the stereo output decreases. The decrease of stereo separation improves a signal to noise ratio. Thus, the referenced patent changes stereo separation in accordance with a noise component included in the detected output, thereby to provide for a required minimum signal to noise ratio. However, the referenced patent fails to disclose any means for removing such distortion component as a multi-path distortion, to which the present invention is directed. Furthermore, the referenced patent requires two control signal paths including a bandpass filter and a high pass filter and for this reason a circuit configuration of the referenced patent is relatively complicated.